Virtual dB & Real Results

For a while now I’ve been abstractly aware of something that I’ve wanted to flesh out graphically.

Specifically, it involves the necessity – and advantages – of precisely zero-beating a station, as opposed to a more relaxed approach of just “good enough”.

It explains the logic of how skilled QRP ops work through pile-ups when the majority of their fellow DXers in the pile-up are QRO. Luck & patience are valuable commodities but technique is worth watts beyond the QRPer’s transmit output.

It also explains the How’s and Why’s of antenna-deficient stations (like mine!) getting through pile-ups quickly when conventional wisdom says they ought not to, given all the Yagis and kilowatts in the same pile-up.

Lack of technique effectively costs watts – again, in a Real way – and often renders QRO operators as “Unintentional QRPers”.

To illustrate this and to see its scope, I injected a reference signal into the antenna jack of my K3. I then zero-beat that signal and dialed my selectivity down to its minimum of 50 Hz to simulate how a rare DX station might adjust his receiver in order to cope with a large pile-up.

Then, to simulate the numerous stations calling him with varying degrees of accuracy in their zero-beat skills, I changed the freq of the injected signal in 10 Hz increments, up & down, and measured the corresponding decrease in the receiver’s audio output in decibels.

Finally, I graphed this offset error vs. loss of amplitude in dB and then converted the corresponding decrease to effective watts based on a theoretical 100-watt transmitter.

Each receiver will respond differently and that response will depend upon, among other things, the selectivity dialed in by the DX station. Many DXpeditions use K3′s and that happens to be the rig that I have handy so that’s what’s represented here.

The X-axis represents departure in Hz of the 10.110.000 MHz RF signal injected into the K3.

A few points:

100 watts attenuated by 13 dB is 5 watts. This occurs only 40 Hz either side of zero-beat. For most people, 40 Hz is very hard to discern by ear. If a 100-watt station is calling 40 Hz from the DX station’s receive freq and a 5-watt QRPer is calling at zero-beat, they’ll both be of equal amplitude in the DX station’s receiver – only their pitch will differ.

The 100-watt op has, by virtue of the distant receiver’s characteristics, made himself a virtual QRPer by being a mere 40 Hz away from the DX station’s receive freq.

Being only 25 Hz away from the DX station’s receiver causes your received signal to be attenuated by 7 dB…..your 100-watt signal now has an effective power of only 20 watts.

Also, 13 dB is a significant (X20) loss (or gain). Look at how much effort we put into maximizing the efficiency of our antennas. Going from a dipole to a Yagi nets us only half that amount. And how much does an amp cost that boosts your transmit power by 13 dB?

At only 50 Hz away, your 100-watt sig has all the punch of 800 milliwatts.

Pretty amazing, right? If that doesn’t illustrate the importance of proper transmit frequency placement when operating split, I don’t know what will.

Granted, these attenuation levels have everything to do with the selectivity of the DX station’s receiver – but you get my point:

Failure to practice precision in where you place your transmit freq negates the labor and cost of such station improvements.

At the date of this posting, there is a thread on eHam about QRP DXing. WG5G may have luck and patience on his side but I bet he knows full well the practical application of the chart above and how to exploit it to his benefit. How else would he have worked 338 countries with 5 watts?!

The next time you work a DX station in a split pile-up, take a listen on your transmit freq to gauge the zero-beat-ability of those still calling. I do this every single time I log a rare station and am always amazed at the absence of callers on the freq the DX has just proven himself to be listening to.

So how to precisely zero-beat in order to maximize the effect of every single watt you’re squirting out?

With the K3, there are four ways:

  • Use the Auto-spot function
  • Use the Spot tone to match the pitch
  • Use the visual LCD indicator
  • Tune to the station using your narrowest (50 Hz) bandwidth

I use a combination of the last two methods knowing that if I can copy the desired station in my 50 Hz passband, that the DX station will hear me when I transmit there.

Again, I often have very little competition within that little sliver of a window.

There are other add-on zero beat indicators and helpers out there but I’ve never had the opportunity to use them and can’t speculate on their accuracy or implementation.

My goal, whether I meet it or not, is to be not just zero-beat with the last station the DX worked, but to ideally be the same pitch in the DX station’s headphones as the station he previously worked. I know that DX stations sometimes tune up or down between QSO’s but I find that if I make them “not have to”, they often won’t.

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